The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, or in ultra-clean or aggressive chemical applications. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the resistive material and also minimizes current leakage during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to a heater controller and an over-mold material that protects the lead-to-resistive circuit interface. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film printing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another process distinct from thin and thick film techniques is thermal spraying, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
Known systems that employ layered heaters typically include a temperature sensor, which is often a thermocouple or an RTD (resistance temperature detector) that is placed somewhere near the film heater and/or the process in order to provide the controller with temperature feedback for heater control. However, thermocouples and RTDs have a relatively slow response time and often “overshoot” the desired temperature. Thermocouples and RTDs are also limited to only detecting an absolute temperature value and thus provide no other independent control.
Other systems often employ “two-wire” control, in which a resistive heating element functions as both a heater and as a temperature sensor, thus eliminating the need for a separate temperature sensor such as a thermocouple or RTD. However, two-wire control systems can have certain disadvantages, such as TCR characteristics of the heating element causing higher wattage at ambient temperatures versus at a set point temperature. Additionally, a heating cycle with two-wire control can be interrupted by the actual temperature detection, and if a short measurement pulse is used, the temperature of the heater may be undesirably increased.
Certain heater systems also employ over-temperature protection, such as thermal switches or bimetallic switches. These systems can be relatively costly and often have a slow response time. Additionally, temperature detection is only local to the actual switch and thus these systems are somewhat limited in their accuracy.